In the semiconductor industry substrates have been joined to one another for years. It is generally necessary to align the substrates to one another before the actual joining process, bonding. The substrates are aligned to one another with alignment systems. The alignment systems are able to detect alignment marks on the substrates and to align the substrates to one another such that the alignment marks complement one another accordingly. In most cases the alignment marks are complementary to one another.
In order to correctly align two substrates to one another, at least four alignment marks, two on each substrate, are necessary. The alignment marks are preferably as near as possible to the edge of the substrate in order to increase the alignment accuracy.
In an alignment process it is always distinguished between the boundary layer surface and the surface of a substrate which is opposite the boundary layer surface. The boundary layer surface is the surface of the substrate which later becomes part of the bond boundary surface. An alignment mark which is located on a boundary layer surface is called a face-side alignment mark, an alignment mark on the correspondingly opposite side is called a back-side alignment mark. Accordingly there are four basic alignment versions, back-to-back, face-to-face, face-to-back and back-to-face alignments.
In back-to-back alignment no serious problems arise since the alignment marks are actually always on surfaces which are freely accessible to all optics, pointed to the outside.
One special challenge in alignment technology is the face-to-face version. In this alignment version the alignment marks are always aligned to the bond boundary surface, therefore the plane between the two substrates. One possibility is transmission detection, transmission measurement and transmission alignment which are only used when all layers on the substrate, including the substrate itself, are transparent to the electromagnetic waves of the electromagnetic spectrum used at the time. This is not always the case for IR radiation and visible light.
The alignment system in patent AT 405775B follows another, third approach. This patent shows that it is possible to cause the front surfaces of the two substrates which are to be aligned to one another to approach one another almost to contact. The upper and the lower substrate are fixed by corresponding sample holders on an upper and lower carriage and can move in translation, on alternate sides out of the alignment station into a measurement station, in which the respective x and y positions of the alignment marks which are the left or right at the time on the respectively upper and lower substrate can be correctly measured. The advantage in patent AT 405775B is mainly that the substrates before and during the alignment process have an extremely small spacing in the z direction, and therefore when the two substrates approach one another they must also only traverse one very short path in the z direction in order to be brought into contact with one another. On this extremely short path the deviation from the desired alignment accuracy in the x and y direction is hardly of any more importance.
One of the most serious technical problems is the positioning faults which arise by the movement of the substrates and which can occur especially in an environment which can be evacuated. Under these conditions air bearings can no longer be used, with which actually a frictionless sliding of the carriages should be ensured.